Seismic prospecting with random injected signal

ABSTRACT

A method of seismic prospecting using an injected seismic signal which includes a series of impulses spaced in time from one another by time intervals of a random nature. The time spacing is substantially non-repetitive and exhibits no ascending, descending or harmonic patterns.

United States Patent [1 1 3,697,938 Taner 1 Oct. 10, 1972 I54| SEISMICPROSPECTING WITH 3,365,697 1/1968 Fail et a1 ..34()/15.5 RANDOM INJECTEDSIGNAL 3,185,958 5/1965 Masterson .34()/15.5 2,671,896 3/1954 De Rosa..343/l8 [72] lnvemo" 2 3 Turban 2,768,372 10/1956 Green ..343/100 [73]Assignee: Seismic Computing Corp., Houston, Primary Exami'fer BenjaminAssistant ExammerN Moskowltz Att0rney-PraveL Wilson & Matthews [22]Filed: Nov. 7, 1969 21 1 Appl. No.3 874,867 [57] ABSTRACT A method ofseismic prospecting using an injected seismic signal which includes aseries of impulses L Cl "340/155 340/15'5 340/55 spaced in time from oneanother by time intervals ofa 1C random nature. The time spacing issubstantially non- [51] Int. Cl. ..G0lv l/22 repetitive and exhibits noascending, descending or I 58] Field Of Search ..340/l5.5 CD, 15.5 TA,harmonic patterns.

15.5 TC,15.5 CC; kg/ 1 8E, 10( CC I 56] References Cited 9 Claims, 7Drawing Figlll'fis UNITED STATES PATENTS Barbier et al. 340/1 5.5

PATENTEDncI 10 m2 F IG. 5

M 7 Tuner INVENTOR BY M, Mae,

WQQ 'L K Uwuee ATTORNEYS FIG. 3A

a 23 22P- E UDZ D/7 Eu an; v 0): 3

FIG 4B F/G.4A

SEISMIC PROSPECTING WITH RANDOM INJECTED SIGNAL Reflection seismographyas used for oil and gas prospecting, has made use of vibratory typesignal sources because of the greater efficiency of coupling and ease ofgeneration of signals, as compared to explosive charges. The vibratorysignals are continuous over periods of time exceeding the travel time ofreflected impulses; in order to obtain time or distance information itis necessary to vary the frequency or otherwise code the injectedsignal. For example, the so-called Vibroseis" method set forth in U.S.Pat. Nos. 2,688,124 and 2,989,726 uses a signal which sweeps from 20 to80 cps in 4 seconds then back in an equal period. This technique hasfound wide utility, but yet lacks some of the advantages of pulse orshot type prospecting. Cross-correlation is necessary, and even soresults in poor resolution of weak reflecting strata when near stronglyreflecting interfaces. For this reason, binary coded sinusoidal signalsas set forth in U.S. Pat. No. 3,221,298 have been used in place ofvariable frequency sources. However, the frequency content of thesesignals in rather shallow, and regular spacing enhances certain unwantednoise and reverberations.

Therefore, a primary feature of this invention is the provision of aninjected seismic signal which will establish no patterns which maycoincide with noise, harmonic relationships, and the like, whereby acontinuous seismic signal may be more readily interpreted with moremeaningful results. Another feature is to provide a seismic signalsource which contains signal pulses of wide frequency band, which at thesame time acquires the advantages of continuous injected signals alongwith compositing or stacking techniques.

In accordance with this invention, an injected signal is used in areflection seismographic method, which signal includes a sequence ofelastic impulses generated by a vibrator or the like as individualpulses rather than a continuous waveform. The time spacing betweenpulses is substantially random and non-repetitive, at least within theaverage travel time for the reflected seismic signals. A large number ofimpulses are transmitted, over a time period exceeding the travel time,providing the advantages of multiple shots, but yet this may beaccomplished in a very short time compared to the total of as manyindividual runs. Not only is time saved, but also the number of datarecordings to be processed is reduced. More significantly, thistechnique results in a composite record which exhibits greatly enhancedsignal-to-noise ratio, and minimizes the side lobes compared to the mainlobe in correlation. The compositing technique used may be simpleadditive correlation, taking into account the same time intervals usedin time spacing the injected signal. The degree of cross-correlationbetween channels using this technique may be expressed as N(Nl )/2,where N is the number of pulses in the sequence; a dramatic enhancementin the number of cross-correlations that will show an increase in themain peak return, compared to noise, is noted.

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asfurther objects and advantages thereof, may best be understood byreference to the following detailed description of an illustrativeembodiment, when read in conjunction with the accompanying drawing,wherein:

FIG. 1 is an elevation view in schematic form of a seismic prospectingsystem which may use the signal source of the invention;

FIGS. 2 A and 2 I! are graphic representations of series of impulsesused for the injected seismic signal according to the invention;

FIG. 3 A is a graphic representation of a recording made in the systemof FIG. I using the injected signal of the invention, and FIG. 3 B is arepresentation of the composited record;

FIGS. 4 A and 4 B are schematic representations of playback arrangementsfor magnetic recordings as may be used with the invention; and

FIG. 5 is a schematic representation of another compositing arrangementproviding the same function as that of FIGS. 4 A and 4 B.

The drawings form a part of this specification and are incorporatedherein. It is noted that like parts appearing in several views of thedrawings will bear like reference numerals.

With reference now to FIG. 1, a schematic representation of a seismicprospecting system is depicted, with the system including a seismicsource 10 for injecting a signal of the type herein described into thesurface of the earth 11, along with a seismometer or geophone 12 fordetecting reflected seismic signals. The pattern of seismic impulsesproduced by the source 10 may be established or controlled by a pulse orsignal generator 13 connected to the seismic source 10. Reflectedsignals reaching the seismometer 12 are converted into electricalimpulses which are applied to a recorder 14 of conventional form. It isunderstood that in place of one seismometer 12, an array of geophonesmay be used according to conventional practice, the output of eachgeophone being separately recorded, or a composite made for a singlerecording. The seismic impulses will travel into the earth and bereflected from various subsurface strata l5 and 16 as indicated. Thetravel time for the impulses will differ depending upon the depth of thestrata and the composition of the material through which the wavetravels, so that following each impulse a series of reflected impulseswill be received at the geophone l2 representing the various strata,reverberations, and of course noise.,

Referring to FIG. 2 A, there is shown a representation of one type ofseismic signal which is injected by the source 10, according to theinvention this signal comprising a sequence of impulses I, I 1,, etc..Each of the impulses I would be of a length of perhaps 4 to 8milliseconds, and of noticeably square shape, i.e., having a somewhatflat wavefonn for a significant period between the leading and trailingedges of the impulse. Thus, the impulse would have a period of constantpressure as distinguished from being a sharp spike rising rapidly inpressure and then dropping rapidly. A square wave of this type producesa wide band of frequencies, including lower frequencies, while stillhaving a predominant high frequency content which provides betterresolution of closely spaced laminated strata. In number, the injectedpulse train of FIG. 2 A may include perhaps 64 pulses spread over atotal time period T of 8 seconds. This time period T is considerably inexcess of the maximum travel time for significant reflected signalsdetected by the geophone, this maximum travel time being ordinarilyconsidered to be less than about 4 seconds.

An important feature of the invention is the nature of the time spacingt,, etc., between the impulses I in FIG. 2 A. These time periods 1 arenon-repeating and define no perceptible ascending or descending pattern.The time spacing may be characterized as pseudorandom, in that thespacing is random within certain limits or qualifications. First, thespacing does not repeat to any significant extent, at least within theaverage maximum travel time to and return from the deepest stratadetected; thus, a given spacing may be duplicated near the beginning andagain near the end of the 8 second period T. Secondly, the minimumspacing will be about 10 milliseconds to provide separation of reflectedsignals, while the maximum spacing will be dictated by the number ofimpulses I to be provided within a given time period T. Also, smallinteger multiples are to be avoided in the spacing periods t, e.g., :40ms, because of the harmonic relationships; this would produce effectssimilar to repeating the impulse spacing. So, in this example, under thelimitations that the spacing t is at least about 10 ms, but less thanabout 200 or 250 ms, and t is not repeated within a 3 or 4 secondinterval nor is a small integer multiple repeated in such interval, thespacing may be considered random.

Instead of the square waveform of the injected signal as seen in FIG. 2A, a series of sharp pulses or a spike waveform may be employed as seenin FIG. 2 B, the difference being in the content of low frequencies. Thewave shape of FIG. 2 B may be more convenient to produce with some typesof vibrators presently available.

The injected seismic signal of FIG. 2 according to the invention may begenerated by any one of several different techniques, employingconventional equipment. For example, an array of small explosive chargesmay be planted, slightly spaced from one another so that one does notset off adjacent ones, and these would be detonated by electricalsignals in the desired time sequence. The source of electrical signalsfor this purpose may be simply a magnetic tape recording having recordedthereon appropriately spaced impulses. Alternatively, anelectrohydraulic vibrator of any one of various types which arecommercially available and described in the literature, may be employed,it being understood that impulse type signals would be used to drive thevibrator rather than a sinusoidal type of signal. A sharp impulse signalis produced by a vibrator if the unit is permitted to bounce rather thanbeing held firmly in contact with the earth. Electrodynamic vibratorsmay also be used. If the minimum cycle time for impulsing one of thesevibrators is greater than needed for signals as indicated in FIG. 2 Aand 2 B, then several vibrators may be used so that they may be pulsedsequentially.

It is understood that the method of the invention may be used foroffshore exploration, as well as for exploration on dry land. In thisevent, air guns may be used as commercially available from BoltAssociates, Inc. of Norwalk, Connecticut. Also, an impulser may be usedwhich is of the type employing a chamber which is filed with explosivegas then ignited, as is well known in this art. The repeat time for adevice of this type is rather long, so a large number would be used togenerate the signal of FIG. 2.

Referring now to FIG. 3 A, a schematic representation of a recording asmade by the recorder 14 in the system of the invention, is illustrated.Although shown as if a visual recording on a strip chart is used, it isunderstood that a magnetic recording would be employed most commonly,and indeed the recording may well be in digital rather than analog form.However, the same principles will apply, as if a visible analogrecording were used. It is noted that the recorded reflected signalsfrom one injected impulse I will overlap and be indistinguishable fromthose for adjacent impulses, since the spacing between impulses is muchless than the average travel time. Thus, a direct observation of therecording is meaningless. The length of one recording will be the periodT, plus an additional period T' to account for the maximum travel timefor reflections associated with impulses I near the end of the period T.The recorded signals in FIG. 3 A are combined and processed as will bedescribed to produce one composite representation as seen in FIG. 3 B,where each of the subsurface strata will be indicated by a clearreflected impulse.

The technique used to derive the representation of FIG. 3 B from therecording of FIG. 3 A may be simple additive correlation, taking intoaccount the time spacing between the impulses I in the injected signal.One scheme for accomplishing this is illustrated in FIG. 4 A, where amagnetic recording device 20 of the drum type is illustrated. Themagnetic tape recording 14, as represented by FIG. 3 A, would bereproduced on the recording surface of the drum 20, so that onerevolution of the drum would equal the [T T'] time period, or onecompleted shot. A large number of pick-up heads 21 are spaced around thedrum, with the spacing between the heads corresponding exactly to thetime periods I t,, etc., between impulses I, 1,, I etc. The same numberof pickup heads 21 would be used as there were impulses I; for theexample given above there would be 64 such heads. The mechanicaldifficulty of positioning a large number of pickup devices around amagnetic playback drum, in view of the necessary close spacing of someof the devices, may require an arrangement as seen in FIG. 4 B, whereinit is noted that a drum 22 is used which has several parallel channelseach of which contains the same recorded information. There isconsiderable flexibility in this case, for positioning of read heads 23.In either event, whether the arrangement of FIG. 4 A or that of FIG. 4 Bis used, the outputs of all of the pickup heads would be applied to asumming device 24, the output of which would be recorded as therepresentation of FIG. 3 B.

Referring to FIG. 5, another technique for compositing the informationon the record of FIG. 3 A to produce that of FIG. 3 B is illustrated.The record produced from the recorder 14 is run through a playbackdevice 25, and the output 26 therefrom is recorded on a magnetic drum 27which is used as a variable delay device. The drum 27 has a pickup head2 B which is variably positioned by mechanical means to produce anydesired delay in the range of the period T. The output from the pickup28 is recorded on another tape device 29. In operation, the tapes 2S and29 would be run in synchronism through 64 successive traverses of therecord, with the delay being changed each time to correspond to theperiods 2,, 1,, etc. This technique may be implemented with morefacility than that of FIG. 4, but requires much longer in elapsed time,to produce the final representation.

Although a time period T of 8 seconds, and a number of impulses I of 64is given above as a convenient example, it is understood that these aremerely illustrative, and other parameters would be used as fit theparticular circumstances.

While the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asother embodiments of the invention, may be apparent to persons skilledin the art upon reference to this description. It is thereforecontemplated that the appended claims will cover any such modificationsor embodiments as fall within the true scope of the invention.

What is claimed is:

l. A method of seismic prospecting comprising the steps of:

injecting a seismic signal, including a plurality of impulses inpseudorandom time spacing from one another, into the earth;

detecting a return signal including reflections of impulses in theinjected seismic signal from subsurface strata in the earth; and

correlating the detected signal according to the pseudorandom timespacing to produce a composite representation of the subsurface strata.

2. A method according to claim 1 wherein the seismic signal exceeds intime duration the maximum effective travel time for reflected impulsesin the detected signal, and wherein a large number of such impulses areproduced within an interval equal to such travel time.

3. A method according to claim 1 wherein the time spacing betweenimpulses is characterized by the absence of small integral multiples oftime spacing one to another.

4. A method according to claim 1 wherein the impulses are characterizedby square waveshapes exhibiting a broad band of frequency content.

5. A method according to claim 1 wherein the detected impulses arerecorded on a single recording and correlation thereof is by additivecompositing in accordance with the individual time spacings betweenimpulses in the pattern.

6. A method of determining travel time between spaced first and secondpoints, comprising the steps of:

transmitting a series of impulses from the first point to the secondpoint, the impulses in the series being spaced from one another in timeaccording to a pseudorandom time spacing;

detecting the series of impulses; and

compositing such impulses in accordance with the pseudorandom timespacing.

7. A method according to claim 6 wherein the impulses are reflected fromat least one third point in travel between the first and second points,and the length in time duration of the series of impulses exceeds theeffective maximum travel time.

8. A method according to claim 7 wherein the number of such impulses inthe series and the spacing in time between impulses IS such that theaverage time spacing is less than the average travel time.

9. A method according to claim 8 wherein the impulses are of a waveshapecharacterized by the absence of substantially sinusoidal portions, andby the inclusion of a broad band of frequency components.

* II i i i

1. A method of seismic prospecting comprising the steps of: injecting aseismic signal, including a plurality of impulses in pseudorandom timespacing from one another, into the earth; detecting a return signalincluding Reflections of impulses in the injected seismic signal fromsubsurface strata in the earth; and correlating the detected signalaccording to the pseudorandom time spacing to produce a compositerepresentation of the subsurface strata.
 2. A method according to claim1 wherein the seismic signal exceeds in time duration the maximumeffective travel time for reflected impulses in the detected signal, andwherein a large number of such impulses are produced within an intervalequal to such travel time.
 3. A method according to claim 1 wherein thetime spacing between impulses is characterized by the absence of smallintegral multiples of time spacing one to another.
 4. A method accordingto claim 1 wherein the impulses are characterized by square waveshapesexhibiting a broad band of frequency content.
 5. A method according toclaim 1 wherein the detected impulses are recorded on a single recordingand correlation thereof is by additive compositing in accordance withthe individual time spacings between impulses in the pattern.
 6. Amethod of determining travel time between spaced first and secondpoints, comprising the steps of: transmitting a series of impulses fromthe first point to the second point, the impulses in the series beingspaced from one another in time according to a pseudorandom timespacing; detecting the series of impulses; and compositing such impulsesin accordance with the pseudorandom time spacing.
 7. A method accordingto claim 6 wherein the impulses are reflected from at least one thirdpoint in travel between the first and second points, and the length intime duration of the series of impulses exceeds the effective maximumtravel time.
 8. A method according to claim 7 wherein the number of suchimpulses in the series and the spacing in time between impulses is suchthat the average time spacing is less than the average travel time.
 9. Amethod according to claim 8 wherein the impulses are of a waveshapecharacterized by the absence of substantially sinusoidal portions, andby the inclusion of a broad band of frequency components.